# 4: Multiplicative Number Theoretic Functions

- Page ID
- 8844

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)

In this chapter, we study functions, called multiplicative functions, that are defined on integers. These functions have the property that their value at the product of two relatively prime integers is equal to the product of the value of the functions at these integers. We start by proving several theorems about multiplicative functions that we will use later. We then study special functions and prove that the Euler \(\phi\)-function that was seen before is actually multiplicative. We also define the sum of divisors and the number of divisors functions. Later define the Mobius function which investigate integers in terms of their prime decomposition. The summatory function of a given function takes the sum of the values of \(f\) at the divisors of a given integer \(n\). We then determine the Mobius inversion of this function which writes the values of \(f\) in terms of the values of its summatory function. We end this chapter by presenting integers with interesting properties and prove some of their properties.

- 4.2: Multiplicative Number Theoretic Functions
- We now present several multiplicative number theoretic functions which will play a crucial role in many number theoretic results. We start by discussing the Euler phi-function which was defined in an earlier chapter. We then define the sum-of-divisors function and the number-of-divisors function along with their properties.

## Contributors

Dr. Wissam Raji, Ph.D., of the American University in Beirut. His work was selected by the Saylor Foundation’s

for public release under a Creative Commons Attribution (**Open Textbook Challenge**) license.**CC BY**